Many current clocking circuits are available. One type of clocking circuit can provide a clocking signal having a variable frequency. Such variable frequency clocking circuits, also referred to as voltage controlled crystal oscillators, are used in many commercial applications, such as in wireless communications devices. While such variable frequency clocking circuits have many useful applications due to the programmable nature of the frequency, these circuits typically have a relatively high variability in the nominal frequency of oscillation due to process variation of integrated circuit capacitance inherent in the manufacturing process of semiconductive devices.
Another type of clocking circuit is a fixed frequency oscillator. Such fixed frequency oscillators do not offer programmable frequency clocking signals but do provide a clock signal with higher accuracy than the variable frequency clocking circuits. One reason for the higher accuracy is that these circuits use capacitors that are separate from the integrated circuit, i.e. off-chip capacitors.
Certain commercial applications would benefit from or may be enabled by a method and apparatus for providing a variable frequency clocking signal that also has higher accuracy at its nominal frequency. One such application is asynchronous digital subscriber loop (ADSL) systems that are under development in the data communication technology area. For example, a technology standard "Network and Customer Installation Interfaces of the Alliance for Tellecommunications Industry Solutions for the ADSL Metallic Interface" (T1E1.413) requires a frequency tolerance of at most +/-50 parts per million with software selectability of frequency in response to a far-end modem request.
Accordingly, there is a need for an improved method and apparatus for providing a clocking signal with variable frequency and high accuracy nominal frequency.